BAGHDAD - A video posted online Saturday appears to show the leader of the Islamic State extremist group that has overrun much of Syria and Iraq delivering a sermon at a mosque in Iraq, in what would be a rare - if not the first - public appearance by the shadowy militant.

The video was released on at least two websites known to be used by the group, but it will take more careful analysis to prove the person shown was indeed the group's leader, Abu Bakr al-Baghdadi. It bore the logo of al-Furqan, the group's media arm. And the face did seem to fit one of the only known photographs of the man, reports CBS News correspondent Charlie D'Agata.

Through brute force and guile, the Islamic State group has seized control of a vast swath of land straddling Syria and Iraq, and has declared the establishment of an Islamic state, or caliphate, in those territories. It proclaimed al-Baghdadi the leader of its state and demanded that all Muslims pledge allegiance to him.

"The mujahedeen have been rewarded victory by God after years of jihad, and they were able to achieve their aim and hurried to announce the caliphate and choose the Imam," he says in the video, referring to the leader.

"It is a burden to accept this responsibility to be in charge of you," he adds. "I am not better than you or more virtuous than you. If you see me on the right path, help me. If you see me on the wrong path, advise me and halt me. And obey me as far as I obey God."

He is dressed in black robes and a black turban, has dark eyes, thick eyebrows and a full black beard. He speaks eloquent classical Arabic, but with little emotion.

The mosque has several dozen men and boys standing for prayer, and a flag of the black Islamic State group is hoisted in the mosque. One man stands guard, with a gun holster under his arm.

At the beginning of the video, the man purported to be al-Baghdadi slowly climbs the pulpit in the great mosque in Iraq's second-largest city of Mosul, which al-Baghdadi's group captured last month. Then the call to prayer is made as he cleans his teeth with a miswak, a special type of stick that devout Muslims use to clean their teeth and freshen their breath.

A senior Iraqi intelligence official said that after an initial analysis the man in the video is believed to indeed be al-Baghdadi. The official said the arrival of a large convoy in Mosul around midday Friday coincided with the blocking of cellular networks in the area. He says the cellular signal returned after the convoy departed.

The official spoke on condition of anonymity because he was not authorized to brief the media.

A Mosul resident confirmed that mobile networks were down around the time of Friday prayers, and then returned a few hours later. He spoke on condition of anonymity because of fears for his safety.

Al-Baghdadi was born in Iraq in 1971. In 2005, during the U.S. occupation of Iraq, U.S. soldiers arrested him. But when the American military closed its Camp Bucca prison, al-Baghdadi was handed over to Iraqi security forces, who let him go, reported CBS News correspondent Elizabeth Palmer.

In the five following years, he built a fighting force that joined Syria's civil war and took over hundreds of square miles.

In Syria, al-Baghdadi honed his skills as a commander with violence so extreme that even al Qaeda's leader Ayman al-Zawahiri cut all ties to ISIS.

That was fine with al-Baghdadi, whose organization says he's the new global leader of extremist jihad and even cataloged its accomplishments in, believe it or not, an annual report.

That listed, among other things, 1,083 assassinations and 537 bombs in parked cars

Continuing that advanced media strategy, which has included aggressive use of social media, ISIS on Saturday released a new English-language magazine, Dabiq. The 26-page first edition is somewhat reminiscent of al Qaeda's own English-language publication, Inspire.

Another aspect of the rule al-Baghdadi envisions was made clear in a series of images that emerged online late Saturday showing the destruction of at least 10 ancient shrines and Shiite mosques in territory his group controls.

The 21 photographs posted on a website that frequently carries official statements from the Islamic State extremist group document the destruction in Mosul and the town of Tal Afar. Some of the photos show bulldozers plowing through walls, while others show explosives demolishing the buildings in a cloud of smoke and rubble.

Showing such ruthless destruction is part of the media campaign designed to show the group is unstoppable, reports D'Agata.

Residents from both Mosul and Tal Afar confirmed the destruction of the sites.

The Islamic State group seized Mosul, Iraq's second-largest city, in June in the opening act of its lightning offensive that sent much of the Iraqi army scattering. Shiite militiamen and volunteers have had to fill the void as the regular army struggles to regroup.

On Saturday, Iraqi Prime Minister Nour al-Maliki removed the chief of the army's ground forces and the head of the federal police from their posts as part of his promised shakeup in the security forces following their near collapse.

Military spokesman Lt. Gen. Qassim al-Moussawi says Prime Minister Nouri al-Maliki signed the papers to retire Lt. Gen. Ali Ghaidan, commander of the army's ground forces, and Lt. Gen. Mohsen al-Kaabi, the chief of the federal police. Al-Moussawi says both men leave their jobs with their pensions. No replacements have been named.

Last month, al-Maliki retired three generals who had been deployed in Mosul and ordered legal proceedings against them. He also dismissed a brigadier general and ordered his court martial in absentia. He said he planned to retire off or court martial more senior officers, but gave no details.

Al-Maliki has also vowed to bring the full weight of military law, including the execution of deserters, on anyone who is found out to have fled the battle.

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